Much of the work in the creation thermal motors has focused on mechanisms which exploit actions, or movements, which occur when thermal materials such as bi-metals, or shape-memory alloys are successively warmed and then cooled.
Some mechanisms have been aimed at replacing the functions of electromagnetic motors, actuators or solenoids, particularly in situations which require compact size or relative low-weight. In part, such devices have attempted to exploit the advantages of devices made using thermal materials which, while providing equivalent force, can often be lighter or smaller than conventional electromagnetic motors or solenoids with bulky windings and heavy magnets or cores. These devices have often employed bi-metals or memory-metal wires, springs, rods or strips which, when heated, move a rigid, pivoted lever or gear. Then upon cooling of the thermal material, the rigid lever, or gear, is returned to starting position by a biasing means-often a conventional extension spring-acting opposite to the thermal material's direction of force. In some of these devices, memory-metal actuator wires, functioning as transducers, are ohmically warmed using electric current as a power source and ambient air as the cooling means.
A further refinement of such devices occurred with the addition of electronic circuits which could control the timing of electric currents activating the shape-memory transducers. Still further improvements occurred with the development of devices employing secondary levers or gears to amplify and transform relatively small movements of the bi-metal or memory-metal transducers in mechanical assemblies.
While such devices provide some measure of usefulness, nearly all are application specific, i.e., in nearly all cases, new mechanisms must be designed "from the ground up" for each particular end use. In comparison, readily-available "off-the-shelf" electromagnetic motors and solenoids enable designers and engineers to quickly develop a multiplicity of mechanisms to suit an individual task or application.
Moreover, many devices employing thermal materials have been relatively difficult and expensive to manufacture. For example, devices employing pivoted levers, gears or other rigid elements connected to memory-metal actuator wires often must be made with exacting part tolerances since small mismatches (e.g. "backlash" between gears, or pivots and pivoting members) would otherwise waste much of the shape-memory materials' short "stroke," which is typically only 3 to 7 percent of active length. Additionally, many devices provide essentially fixed torque though out their "power-strokes" when it is desirable to have a responsive capability since it is often necessary to overcome relatively greater force at only start of a cycle or to respond to temporary increase demand for a greater torque for a small part of operating cycle. Consequently, many of the devices have been inefficient in the use of relatively expensive shape-memory-material.
Also, some devices employ separate, costly strain-reliefs to avoid over-stressing or breaking shape-memory elements if mechanism travel becomes blocked or restrained during operation. Other devices make no provision for strain relief at all.
In addition, many battery-powered devices have had limited operating lives. These mechanisms are most often made of rigid moving parts with relatively high mass acted upon by thin, fast-acting memory-metal wires. Consequently, these mechanisms exhibit substantial inertial resistance or fail to absorb the shock of a "power stroke" leading to stress fatigue and breakage of thin actuator wires after relatively few operating cycles.
Therefore, it is desirable to provide improved actuators and motor-like devices which are simple and relatively inexpensive to manufacture and yet can function for extended periods of time in a multiplicity of applications. More specifically, it is desirable to provide simple motors and devices employing cantilevered, resilient, shock-absorbing means in a efficient "pivotless" transducer. It is desirable that such devices be also lightweight and flat while at the same time be resistant to damage when operated, even when mechanism travel is blocked or hindered. Additionally, such devices should be efficient in the use of shape-memory materials and be capable of appropriately varying torque in response to load requirements during device operation.